Refractory materials are designed to retain their strength at high temperatures and are thus used, for among other things, as liners in furnaces, incinerators and ovens. The refractory materials may be applied in a number of known ways including by casting, pumping, shotcreting or dry gunning.
Although casting is suitable for creating liners in many applications, it requires significant time and expense in creating and using forms for the casting. Furthermore, it is not always feasible to build forms depending on the shape of the casting and or its application.
Pumping a wet castable through a hose also has problems. Among other things, pumping a wet material through equipment can often to lead to plugged lines. For example, variations in the raw materials and/or conditions at the job site can lead to a marginally pumpable material that can cause plugging and setting of the material in the hoses, thus shutting down the job. Additionally, pumping requires the use of expensive equipment.
While shotcreting, which involves pneumatically projecting mortar or concrete onto a surface at a high velocity, may work, it requires the use of expensive equipment and a large work crew to operate. Shotcreting introduces a setting agent at the nozzle, where it is not evenly distributed throughout the refractory mass, thereby leading to variations in material properties. In order for shotcreting to be used with colloidal silica bonded materials, it is generally necessary to use 9-10% colloidal silica, thereby resulting in 6% to 6.6% water in the finished product. To prevent the castable from slumping on the wall, a water-containing setting agent is injected at the nozzle to cause the mass to harden upon being applied to the surface.
In addition to the problems set forth above, the increased level of water required in the pumping and shotcreting methods in order to achieve a fluid consistency results in a higher porosity product. This results in decreased properties and resistance to slag and metal penetration. Additionally, the high percentage of water also increases the drying time for these methods.
Current dry gun methods include the introduction of water into a nozzle. The nozzle being attached to the end of a hose used to transport the dry material in a compressed air stream. However, problems have arisen using this process from, among other things, the introduction of water. In particular, it is important to make sure that the cement is properly hydrated so a large amount of water is necessary to be introduced at the nozzle. The use of a large amount of water weakens the compressive strength of the finished product. Additionally, it is very difficult to wet the fine components of the material in the very short residence time within the nozzle. The poor mixing between the fine powders, coarse aggregates and water in the nozzle results in some portion of the material bouncing off the surface or rebounding. This creates clean up, occupational hazards, and waste as well as increasing the cost of application while decreasing the processes efficiency. Dry gunniting also creates a significant amount of dust that can be hazardous to operators of the machines.